It is now common practice to explore the oceans of the earth for deposits of oil, gas and other valuable minerals by seismic techniques in which an exploration vessel imparts an acoustic wave into the water, typically by use of a compressed air "gun." The acoustic wave travels downwardly into the sea bed and is reflected at the interfaces between layers of materials having varying acoustic impedances. The wave travels; back upwardly where it is detected by microphone or "hydrophone" elements in a streamer towed by the vessel to yield information regarding characteristics of the underwater material and structures.
A towed streamer comprises a plurality of pressure-sensitive hydrophone elements enclosed within a waterproof jacket and electrically coupled to recording equipment onboard the vessel. Each hydrophone element within the streamer is designed to convert the mechanical energy present in pressure variations surrounding the hydrophone element into electrical signals. Due to its often extreme length (on the order of kilometers), the streamer is divided into a number of separate sections or "modules" that can be decoupled from one another and that are individually waterproof. Individual streamers can be towed in parallel through the use of paravanes to create a two dimensional array of hydrophone elements. Data buses running through each of the modules in the streamer carry the signals from the hydrophone elements to the recording equipment (so-called "acoustic data").
In addition to acoustic data, it is also important to collect and transmit data concerning operational status of the array to the vessel (so-called "nonacoustic data"). Nonacoustic data comprises physical characteristics of interest regarding the operation of each module, including whether water has invaded a module in the streamer, module temperature, module depth and power supply voltage.
Today, many towed arrays have digital data channels. The primary advantage of digital data transmission is its ability to handle a significantly greater number of sensors for a given streamer size. For example, streamers with a thousand or more. sensors would be of an impractical large diameter to simply contain the analog bus conductors therein. Also, with digital data transmission, data transmission rates are higher and, with proper attention to shielding of electromagnetic interference, data fidelity is maintained from the hydrophone to the recording equipment.
For instance, U.S. Pat. No. 3,996,553, that issued on Dec. 7, 1976 is directed to a plurality of data acquisition units connected to a central signal processor through a common digital telemetry link. The telemetry link includes a data channel, an interrogation channel and a control channel. The central signal processor sends an interrogation signal through the interrogation channel to the data acquisition units. As each data acquisition unit recognizes the interrogation signal, it transmits its acquired data back up to the central processor through the data channel. Any selected data acquisition unit, when it receives a control signal through the control channel at the same time that it receives an interrogation signal through the interrogation channel, can be caused to perform a function different from all other units. The signal propagation velocity through the control channel is different from the signal propagation velocity through the interrogation channel. One of the two signals may be transmitted through the faster channel at a selected time later than the other of the two signals is transmitted through the slower channel. The selected time difference between the transmission of the two signals is proportional to the ratio of signal propagation delay difference between channels. Accordingly, the signal propagating through the faster channel will overtake and intercept the signal propagating through the slower channel at the selected data acquisition unit. Each data acquisition unit may have one or more input channels. Each input channel is connected in turn to the data channel through a stepping switch or multiplexer. The interrogation signal may exist in one of two or more states. In the first state, the interrogation signal resets the multiplexer, in the second state, the interrogation signal advances the multiplexer to the next input channel in sequence. Thus, this system allows for control signals to command changes of state within the individual multiplexers in the system.
In the past, these data acquisition units were physically housed in metal canisters located between the modules in the streamer. Being metal and containing electronic components, the canisters were relatively heavy. Thus, the canisters were made large (on the order of 4-5 inches in diameter and 15-18 inches in length) and spacious inside to give the canisters an overall near-neutral buoyancy.
As an exemplary teaching of such a canister arrangement, U.S. Pat. No. 4,092,629, that issued on May 30, 1978, is directed to a seismic sensor streamer having 50 modules and much of the seismic data processing electronics decentralized into the cable structure itself. The streamer is coupled to a central station mounted in a recording vehicle. The central station includes recording circuitry and apparatus to receive, process and record digital data words from a data link in the streamer and circuity for transmitting control signals into an interrogation link in the streamer. The electrical output of each sensor unit constitutes a separate input channel. The modules are spaced apart and interconnected by small diameter, cylindrical inter-module canisters that contain a transceiver unit for processing the signals from ten sensor units in an associated module. Contained within each transceiver unit is a multiplexer having a plurality of filtered input channels coupled respectively to the elemental sensor units, and an output. In response to a first interrogation pulse transmitted through the interrogation link from the central station unit, the multiplexer advances to a selected input channel to acquire a first analog data sample. A second interrogation pulse sequences the respective multiplexers in all 50 modules to select a second channel for sampling and digitizing to provide digital data words for the respective second channels. The self-clocking phase-encoded data words transmitted from the respective transceiver associated with each module are ordered in accordance with the propagation delay time of the interrogation link between the central station and the respective transceiver units. Self-clocking data words from corresponding channels within the respective transceiver units are ordered in accordance with the channel-select sequence during a scan cycle.
Unfortunately, the inter-module canisters had several significant disadvantages. First, since they were of large diameter compared to the modules, they created viscous drag and turbulence when the streamer was towed. Not only was the streamer required to be stronger (thus heavier) to handle the extra strain caused by the drag, but the turbulence churned water, distorting the acoustic signals before they reached the hydrophones and impairing signal quality.
Second, since streamers are commonly stowed on large spools (involving bending of the streamer around curved surfaces under tension) when not in use, the inflexible canisters presented storage problems. All long, flexible structures (such as ropes, streamers or bananas) have minimum bend radii beyond which excessive internal stresses may damage the structure, particularly if the structure is subject to axial stress. The canisters created strain on the ends of the modules as the modules were forced to bend and stretch to adjust for the inflexibility and length of the canisters length (15-18 inches). Since they were of large diameter (4-5 inches versus less than 3 inches for the module), they created lumps in the stored streamer. In an effort to alleviate the strain and lump problems, workers had to insert pads or blocks around the canisters as the streamer was wound onto its spool to cushion the modules. The workers also had to remove these pads or blocks when the streamer was deployed from the spool.
Third, the signals emanating from hydrophones are of extremely low level since hydrophones are of high impedance, the pressure signals from the ocean floor are weak and the volume of piezoelectric material in the hydrophones is low for economic and space reasons. Signal attenuation and distortion due to the length of wire, triboelectric noise generated when the wire is flexed due to streamer motions and electromagnetically coupled noise from other wires within the streamer (e.g. digital telemetry data) all contribute to degradation of the hydrophone signal. It is optimal to transmit the hydrophone signals over the shortest possible wire distance. Since a module is typically 50 to 100 meters long, hydrophone signals had to travel, on average, half the length of the module before being processed in the prior art inter-module canisters, degrading signal quality.
Fourth, electronic circuits are typically "active" in the sense that power must be supplied for their proper function. Such circuits dissipate most of their input power as heat, which must be managed so that high temperatures (that tend to shorten the life of electronic components) are avoided. Areas of high circuit concentration (such as the prior art canisters) are, in general, more prone to heat concentration (and resulting high temperatures) than spatially distributed circuitry.
Finally, the inter-module canisters represented yet more parts that had to be assembled to create a streamer and required the use of twice the number of connectors.
To date, deepwater seismic streamers have had a diameter of at least 2.8 inches, although some small streamers of limited capability and employed for specialized work have been of less diameter. This large diameter was necessary to house larger, stronger strain cables and larger diameter hydrophones. This larger diameter posed a storage problem, as such streamers are typically more than 3 km long. The sheer volume of the streamer and handling equipment exacerbated the modern practice of towing multiple streamers in an array. Further, as damaged modules must be shipped to repair sites, the larger diameter posed a shipping problem.
What is needed in the art is a towed hydrophone streamer that eliminates the inter-module canisters, distributing the electronics instead within each module to yield a streamer that is thinner, more flexible, quieter and has fewer connectors.